The invention relates to a .gamma.' (gamma prime) hardened nickel-iron base superalloy.
Liquid metal fast breeder reactors have been designed to incorporate 20% cold worked 316 Series Stainless Steel (SS) for fuel cladding and duct applications. The National Alloy Development Program has as one of its goals the objective of finding materials that may be substituted for 20% cold worked 316 SS in these applications, which substitution materials will have greater resistance to swelling as well as improved strengths. It would be desirable to obtain alloys having these improved properties since they would result in a decreased cost in the power generation cycle as well as reduce the cost for spent fuel handling.
Gamma prime strengthening in stainless alloys is well known to the commercial superalloy industry. Materials such as A-286 and Nimonic PE16 typify this class of materials. A material for use in fuel cladding or duct applications in liquid metal fast breeder reactors has additional constraints and material requirements because of the unique and extreme nature of the neutron irradiation environment. A fuel cladding alloy, for example, will be exposed to flowing liquid sodium on the one side and nuclear fuel on the other side. The neutron irradiation introduces new and novel physical processes which can have a severe impact on the properties and behavior of the structural material. Neutron irradiation has an effect on, for example, the phenomenon of swelling in which the physical dimensions of an alloy will change due to the production of internal cavities, and the phenomenon of irradiation creep, in which an alloy will elastically deform under temperature and stress conditions which would not produce deformation without the irradiation environment. These special problems require special materials.
The liquid sodium environment, although potentially detrimental to many materials has one advantage that was utilized in the conception of the present invention. This advantage is that because of the chemical nature of liquid sodium and the low operating oxygen content of liquid sodium in reactors, it actually shields the materials from oxidation. This removes one restriction which is generally incorporated in normal nickel-iron based superalloys, i.e., the chromium content of those materials are generally higher, for example, in the range 15 to 19 wt%. This higher chromium protects the surface of the material from oxidation. Since materials immersed in liquid sodium in breeder reactors are not exposed to the harsh oxidation environment, lower chromium materials can be designed for reactor applications. The advantages of lower chromium materials include less tendency to form the detrimental sigma phase, potentially better fabricability, and potentially higher swelling resistance.
Low nickel alloys are more valuable than higher nickel materials for breeder applications since nickel has a relatively high neutron absorption cross section. This results in effectively wasted neutrons and reduced power production efficiency.
The alloys of this invention as described herein were designed by uniquely combining the gamma prime strengthening, solid solution strengthening, and silicon as a swelling inhibitor to the low chromium and low to intermediately low nickel range. The concept is contained in the unique combination of the above factors. The actual composition range can be improved somewhat by minimizing the potential phase instabilities commonly observed in nickel-iron superalloys, e.g., the G, sigma, mu, and Laves phases and by optimizing the titanium and aluminum contents and ratios. The titanium and aluminum optimization may be produced by the normal procedure of balancing the increased strength of high volume fractions of gamma prime phase against the decreased fabricability and weldability.